Method and means for improving the spectrum utilization of multichannel telephone systems

ABSTRACT

A system and method for improving the spectrum utilization of voice communications channels by increasing the number of voice conversations a given number of lines can service. This invention can be used to improve the reliability and efficiency of conventional TASI (Time Assignment Speech Interpolation) systems and allow such systems to provide good performance when utilizing a mixture of types of telephone lines; including, microwave, cable, and satellite, and two wire configuration lines.

RELATED APPLICATION

This application is related to my co-pending application filed July 18,1977, Ser. No. 816,661, entitled, "Method and Means for Improving theSpectrum Utilization of Communications Channels".

BACKGROUND OF THE INVENTION

While features of the invention are subject to a wide range ofapplications, the invention is especially suited for use in TASI (TimeAssignment Speech Interpolation) systems and will be particularlydescribed in that connection.

TASI systems have been used to improve the utilization efficiency ofvoice communications systems by reducing the time that telephone linesare temporarily idle. For example, in a conventional two-way telephonecircuit, over 50% of the line's capacity is wasted, accommodating thelistener's transmit channel.

The TASI system constantly monitors speech channels and quicklyreassigns lines from idle channels to active channels increasing theoverall efficiency of the system. TASI systems have been described inthe literature; for example, K. Bullington and J. M. Fraser,"Engineering Aspects of TASI", BSTJ, Vol. XXXVIII, March 1959, and"Transmission Systems for Communications" Bell Telephone Laboratories,1970, pages 682 to 684 including references.

In the conventional TASI system, separate line(s) are used fortransmitting channel assignment information, although systems have beendescribed wherein the initial channel assignment information istransmitted via the line assigned to transmit the channel.

It is noteworthy that the telephone communications network systemsutilize a multiplicity of transmission systems; including, wire line,wideband cables, Satellite links, microwave radio links, etc., and thatthe time delay characteristics of these transmission systemssubstantially differ. Such significant differences in delay can causedegraded control performance of a TASI system when, say, long time delaylines are used in transmitting control information of short time delaylines. Also, failure of a line transmitting control information cancause a large number of circuits to improperly perform. Thus, aconventional TASI system may be subject to poor operation andsimultaneous interruption of a number of conversations if it utilizes acommon control line.

If TASI systems utilize the assigned line to transmit the initialchannel assignment information, the problem of a difference in timedelay between the line carrying the voice signal and the line carryingthe control information is eliminated except for poor timing of the linedisconnect system. Also, since conventional TASI systems transmitinformation regarding channel assignment status and idle line statusover a separate line, these systems are subject to performancedegradation when the control line becomes inoperative. Of course, sparelines can be provided, but at a loss in line utilization efficiency.Furthermore, if a line carrying a telephone signal fails in suchsystems, a conversation is interrupted until the call is reassigned.

SUMMARY OF THE INVENTION

A general object of the instant invention is to provide means forimproving the spectrum utilization of communications channels.

A further object is to improve the operating efficiency of a TASI typesystem. Another object of the invention is to allow the use of a mixtureof telephone lines having appreciably different time delaycharacteristics to be used in a TASI type system. Also, this inventionallows TASI systems to be used with two wire lines. An additional objectof this invention is to convert intelligible crosstalk intounintelligible crosstalk.

A further object of the instant invention is to improve the reliabilityof the control circuit sensitivity of a TASI type system. A stillfurther object is to provide a system that may be constantly monitoredand cause inoperative lines to be automatically removed from operationand alarm circuitry automatically activated.

The present invention provides improved telephone line utilizationefficiency for multi-channel telephone systems and in one embodimentutilizes the following method steps:

1. Sensing the speech activity of a telephone channel.

2. Selecting a line from a group of inactive lines for transmitting thespeech wave sensed in Step (1), and subsequently connecting the speechwave to the selected line.

3. Rapidly transmitting (in approximately 10 milliseconds) the initialchannel identification information over substantially the entirebandwidth of the selected line to the remote end of the line; and,

4. Transmitting continuous channel verification information in anarrowband slot in the passband of the line concurrently with thetransmission of the speech wave.

In one embodiment of the invention the narrowband slot of Step (4) has abandwidth of approximately 300 Hz and one preferred arrangement would beto have the slot fall between approximately 2,000 and 2,300 Hz.

An added arrangement of the invention would be used to implement theTASI type system wherein two wire transmission line configurations areused. In such applications there is possible degradation of performancecaused by imperfect separation at the junction between two wire and fourwire lines. In one embodiment of the invention the speech is encoded inone speech direction differently than it is encoded in the otherdirection. It is necessary to decode the speech at the receive or farends of the system in order to restore intelligible speech. The overallobject of this procedure is to render crosstalk unintelligible so thatthe crosstalk is less disturbing and privacy is maintained.

In some systems, encoding would be provided only for speech going in onedirection. Thus, encoding equipment would only be supplied for thetransmitting end and the receiving end at the far end of the circuit.Such a system could utilize frequency inversion at the transmit end ofthe circuit. In order to restore the intelligibility of the speech itwould be necessary to supply a complementary frequency inversion at thefar end receive circuit.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription, taken in connection with the accompanying drawings, whileits scope will be pointed out in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of one embodiment of the invention.

FIG. 2 shows, in block and schematic form, the talk or transmit end ofthe invention.

FIG. 3 shows, in block and schematic form, the listen or receivecircuitry of the invention.

FIG. 4 is a block diagram of a frequency inverter suitable for use inone embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a simplified block diagram of one end of a system using anembodiment of the invention.

The T line, or talk line, feeds block 100(A) the Channel A transmitter.Each channel transmitter incorporates circuitry for locating an unusedtransmit line, switching circuitry for connecting the channel to any oneof the available lines and a channel identification wave generator.Details of the channel transmitter circuitry are shown in FIG. 2, anddescription of this equipment, as well as the idle signal generatorcircuitry, is provided below.

The output lines of the channel transmitters are connected throughsummation circuits, not shown, to line hybrid circuits which are usedfor isolating the talk and listen signals when two wire lines are used.Hybrid transformers are commonly used for providing such isolation.Unfortunately, it is difficult to achieve and maintain high degrees ofisolation.

It is an important advantage of the present invention that it reducesthe effects of poor talk-listen isolation. For detailed discussions ofhybrid circuits, two wire and four wire lines, one can consult a numberof publications dealing with telephony, including the above cited book"Transmission Systems for Communications", written and published by BellTelephone Laboratories, 4th Edition, 1970.

In order to reduce the deleterious effects of crosstalk caused byimperfect hybrid circuits or other imperfections, the transmitter at oneend of the circuit may be equipped with encoding devices such as afrequency inversion system, and at the other end a compensatingfrequency inversion system is incorporated into the receiver.

Each of the n lines feeds a line receiver which includes means fordecoding the m channel (m is greater than n) identification waves andusing the decoded information to switch the received speech wave to theassigned channel. Also included in the receiver is circuitry foractivating an alarm if the line fails. FIG. 3 as described below, showsdetails of the line receiver.

Also shown in FIG. 1, is the line activity indicating multiwire bus 101which provides information to the various channel transmitters allowingthe transmitter to make proper line assignments.

As shown in FIG. 1, hybrid circuit 300(1) feeds the line receiver, andisolates the receive signal from Channel A transmit signal.

FIG. 2 shows one type of transmitter suitable for implementation of thisinvention. The Channel A T, talk circuit, feeds speech presence detector104A. This speech detector may utilize various type circuits, includingthe type disclosed in U.S. Pat. No. 3,337,808. The control voltageproduced by speech detector 104A feeds stepper control circuit 116A. Onthe initiation of a speech presence indication from 104A, steppercontrol 116A causes switches 118A and 120A to start stepping from oneswitch position to another until an idle talk line is located.

One possible procedure for locating an idle line is shown in FIG. 2.Switch 120A, as well as 118A, is preferably an electronic switchingcircuit and may use integrated circuit gates. For simplicity ofexplanation, a mechanical type switching circuit is shown. Actually,some equipment designers may prefer to use mechanical switches. However,for rapid operation, electronic switches are preferred. Each contact ofthe switch assigned to a line is connected through a resistor to +E voltpoint. For example, the contact representing line 1 is connected to +Ethrough resistor 10, and the contact representing line 2 is connected to+E through resistor 20, etc.

The arm of the switch is returned to ground through resistor 122A. Itshould be noted that the resistors 10, 20, 30, etc. are connected incommon with all of the channel transmitter's switches 120A to N.Therefore, if line 1 is in use, the channel using line 1 connects itsresistor 122 to resistor 10 reducing the voltage appearing at thecontact side of resistor 10. This reduced voltage is sensed at allchannel 120 switches, causing any stepper circuit in operation tocontinue to step past the line 1 position. When, say, 120A switch iscaused to step to the line 2 position, which, for example, is idle inthe talk direction, the voltage sensed across resistor 122A would besubstantially higher than when switch 120A was in the line 1 position.For example, if resistors 10, 20, 30, etc. are 10,000 ohm resistors, and122A to N are 1,000 ohm resistors, and +E is 10V, there would be 0.909volts across resistor 122A in the line 2 position, and 0.476 voltsacross resistor 122A in the line 1 position.

These differences in voltage allow the stepper circuit to stop the lineswitch in the first inactive position. In addition the voltagedifferences control the transmission of the idle signal over thetemporarily idle lines.

The line associated 120 contacts are connected together by line activityindication bus 101.

Stepper control circuit 116A also controls the signal switching circuit118A. Accordingly, if switch 120A stops at line 1, so does switch 118Aconnecting the signal circuit to line 1. The voice signal from Channel Atalk circuit is processed as follows:

The voice wave appearing on the Channel A T line, besides feeding thespeech presence detector 104A, feeds time delay circuit 102A. Thiscircuit delays the speech wave so as to allow some time for the channelassignment circuitry to operate at the far end of the circuit. A timedelay in the order of 10 to 20 ms would be required for typicalinstallations.

For these values of time delays, the equipment designer has the choiceof using mechanical magnetic tape time delay loops or solid state timedelay circuits. For high reliability performance, solid state chargecoupled integrated circuits are a good choice. Such devices as theSAD-1024 analog delay line as manufactured by the Reticon Corporation ofSunnyvale, California, are available for such applications.

The time delay circuit serves two purposes.

(a) It delays the voice signal so that little or no initial speechsounds are lost after speech presence is detected and while thechannel/line assignment is being established and,

(b) It avoids the transmission of speech during the transmission of thehigh speed channel identification signal reducing the probability oferrors in connecting lines to the proper channel utilization circuit.

It is possible to delete the time delay circuit, but, in this case, agating circuit should be provided in order to gate the speech wave offduring the transmission of the initial identification signal so as toavoid causing channel switching errors. Also, if time delay circuits arenot provided, a finite clipping of initial voice signals will besuffered.

The output of time delay circuit 102A is amplified, if necessary, inamplifier 108A, which, in turn, feeds band reject filter 110A. Thisfilter cuts a narrow slot in the passband of the voice signal in orderto provide spectrum space for the narrowband channel identificationsignal. In a prior patent, U.S. Pat. No. 3,684,838, it was disclosedthat a cut of 200 or 300 Hz at preferably the mid-upper range of thetelephone transmission channel did not materially degrade quality orintelligibility of speech waves. For example, a band reject filter whichsubstantially attenuates speech components between 2,000 and 2,300 Hzwould be suitable for this application. The output of filter 110A feedssummation circuit 112A. Also feeding summation circuit 112A is Channel Acode generator 106A. This generator produces two types of identificationwaves;

(a) a high speed channel identification wave which may use substantiallythe entire line's bandwidth, typically between 400 and 2,700 Hz, and

(b) a narrowband channel identification wave which must fall within theslot produced by band reject filter 110A, for example, 2,050 to 2,250Hz.

As to the type of identification wave used, the designer may use any ofthe numerous signalling methods; such as, on/off keying, frequency shiftkeying, and phase shift keying.

Such systems are detailed in numerous publications; for example, W. R.Bennett and J. R. Davey, "Data Transmission", McGraw-Hill, New York,1965. The code generator, when initially sensing speech presence fromthe control voltage produced by speech presence detector 104A, producesa high speed Channel A identification signal. This signal would requireapproximately 10 ms for transmission, and would start some 2 ms afterspeech is detected by detector 104A. The 2 ms interval is provided toallow the idle line locator to select an idle line.

There are numerous code generators available to the designer. Forexample, the book "Digital Integrated Electronics", H. Taub and D.Schilling, McGraw-Hill, New York, 1977, in Chapter 10, discussesequipment useful for such purposes and also provides design information.

It is possible to use a single code for both the high speed andnarrowband channel identification waves and merely slow down the readoutspeed by reducing the clock frequency when transmitting the narrowbandwave. It is desirable to simultaneously shift the carrier frequency ofthe keyed wave to be sure to center the keyed wave in the passband ofthe narrowband filter when transmitting the narrowband identificationwave from a frequency centered in the line passband for the high speedtransmission.

When the transmission of the high speed channel identification signal iscompleted, the Channel A code generator initiates transmission of thenarrowband Channel A identification wave, which continues until thespeech detector 104A indicates that the local A speech channel has beenidle, for, say, 300 ms. At that time the associated line is disconnectedfrom Channel A and generator 106(A) can cease operation.

When Channel A talk circuit is idle, line 2 should, during high trafficactivity conditions, be freed for service with other channels. Thedisconnecting of line 2 from Channel A is accomplished as follows:

The speech presence detector 104A will produce a no signal indicationvoltage which will, after, for example, 200 to 300 ms, cause steppercontrol circuit 116A to control switching circuits 118A and 120A toswitch to their idle positions. Thus, Channel A is disconnected fromline 2.

It is advantageous that means be provided for monitoring theavailability of lines at all times. Accordingly, additional circuitry isprovided for transmitting an idle code signal whenever a line isunassigned.

This continuous protection is provided by using the rise in voltagewhenever all of the arms of the various 120 switching circuits aredisconnected from the line activity sensing resistors 10, 20, 30, etc.,to control associated gates. For example, if line 2 becomes inactive,the voltage at the switch contact end of resistor 20 rises. Thisincreased voltage is sufficient to close gate 50 passing the idle codewave generated in generator 60 to summation circuit 80. When line 1 isidle, the idle wave passes through gate 40 to summation circuit 70.Thus, means are provided to insure constant monitoring of the lines'condition. The idle signal may have characteristics similar to thenarrowband channel identification waves.

The output of summation circuit 112A is amplified in amplifier 114A. Theoutput of amplifier 114A is fed to a conventional frequency invertercircuit 126A for transmitters at one end only of the system. Thefrequency inversion converts high audio frequency components to lowfrequency and vice-versa. By this procedure, crosstalk signals are madeunintelligible.

Thus, by this procedure, the "talk" waves going in one direction areunintelligible to local "listen" paths. Also, as will be discussed inthe section treating FIG. 3, the receiver inverts the desired receivespeech wave and accordingly causes any local talk crosstalk speechsounds due to the hybrid circuit unbalance to be unintelligible. Sinceunintelligible crosstalk is normally less disturbing to conversations,this is a major feature of the invention. Furthermore, because thepresent system will often be used to improve the performance oftelephone communications in a single organization, where many users ofthe telephone facilities would be most disturbed if co-workers were ableto hear their conversations, the elimination of intelligible crosstalkis a most significant advantage of the system.

It should be noted that since the frequency inversion system is acomplementary transformation, it is important that frequency inversionshould be used in the talk circuit at only one end of the system. Forexample, if, in an East/West system, the talk frequency inversion isprovided for the East talk circuit, none should be provided in the Westtalk circuit and frequency inversion be applied only to the West receivecircuit.

Details of one type of frequency inversion circuit are discussed belowin the description of FIG. 4. Switch 124(A) should be open in the Eastlocation, and closed, disabling inverter 126(A) in the West location.

It will be apparent to those skilled in the communications art thatother types of speech privacy systems may be used for convertingintelligible speech to unintelligible speech, including a methoddisclosed in U.S. Pat. No. 2,880,275.

The output of inverter 126A or switch 124A feeds line switching circuit118A to the selected line's hybrid circuit causing Channel A speech waveto be transmitted to the far end of the circuit. The other channel wavesare processed in the same manner by their channel transmitters.

If a gate is used in lieu of time delay circuit 102A, or if aninsufficient length delay circuit is used, the gate circuit can becontrolled by speech detector circuit 104A or by an idle contact ofswitch 120A.

FIG. 3 shows the details of the receive or listen equipment. As is trueof the transmit circuits, two receive circuits are assigned to each lineused; one, at say the East end, and one to the West end. The basic taskof the receive unit is to decode the transmitted channel/line assignmentcoded wave and accordingly switch each line to its assigned channel.Another basic function of the receivers at one end of the system is tofrequency invert the previously inverted speech wave to restoreintelligibility. Another important task of one preferred receiverembodiment is to continuously monitor the availability of the lines soas to cause prompt alarm activation if a line becomes unavailable.

Referring to FIG. 3, the receive port of the line 1 hybrid circuit feedsbandpass filter 204(1) through the arm of relay 202(1). The relay isshown in the position proper for reception of either a narrowbandchannel identification wave or an idle channel signal. The output offilter 204(1) selects the narrow signal wave in the range of; forexample, say 2,000 to 2,300 Hz, and attenuates the speech componentsfalling below, 2,000 Hz and above 2,300 Hz. Filter 204(1) feeds channelsignal detector 206(1). The channel signal detector decodes the channelassignment signals and provides a control signal for the switchingcircuit 214(1). Switch 214(1) connects the received voice wave from line1 to the assigned channel. The example illustrated in FIG. 3 is that ofline 1 connected to Channel B.

Also connected to the output of filter 204(1) is idle signal detector208(1). When the idle signal detector receives an idle signal twoeffects are caused to occur. First, switch 214(1) disconnects line 1from whatever channel it had been connected to and switches the signalpath to the idle or off position. And the second effect is the operationof relay circuit 202(1) which switches the signal input from filter204(1) to directly feed channel signal detector 206(1). Thus, thechannel signal detector is fed the entire incoming channel bandwidthrather than a small frequency segment.

Accordingly, the receiver is ready to receive a wideband, high speedchannel identification signal.

Thus, the receiver is designed to recognize the presence of three typesof control signals; i.e.,

(a) the high speed channel assignment signal,

(b) the narrowband channel assignment signal, and

(c) the idle signal.

Since at least one of the above signals is always transmitted, absenceof all of them activates line failure detector 210(1), which in turnenergizes an alarm circuit. The line failure circuit, instead ofdetecting the 3 signals directly, may be connected to detectors 206(1)and 208(1), in order to determine if a control signal is present.

It should be noted that in addition to its use in the line failureindication system the narrowband channel assignment signal allows theline to switch to the desired channel when there is an error intransmission of the high speed channel assignment signal. While such anerror will cause the loss of some of the speech wave, the channel willbe eventually connected. For a typical narrowband signal, instead of 10ms, it will take approximately 100 ms to establish the circuit.

The input line also feeds a frequency inverter, which is used to restoreintelligibility of inverted speech waves.

If the receiver is located at the end of the circuit not requiringfrequency inversion, switch 218(1) is switched to the bypass position,disabling the frequency inversion. The speech wave is then passedthrough a band reject filter 212(1) which substantially removes thechannel assignment coded signal wave from the speech wave which fall,for example, between 2,000 to 2,300 Hz. The output of filter 212(1)feeds resulting speech wave to the channel switching circuit 214(1)which in turn feeds the speech wave to the assigned channel.

FIG. 4 shows one example of encoding talk signals by frequency inversionas shown in block 126.

This same circuit may be used at the other end of the system fordecoding the signal by complementary frequency inversion as shown inblock 216.

The speech wave is amplified in amplifier 402 which may be required toprovide a more suitable level of impedance. The output of amplifier 402feeds Balanced Modulator A 404. Also feeding Balanced Modulator 404 isOscillator A block 406. The operating frequency of Oscillator A isselected so that one of the sidebands of the double-sideband waveproduced in Modulator 404 falls in the passband of single-sidebandfilter 408. As an example, the oscillator can be set to 100 kHz, and forgood stability should be of the crystal controlled type.

Thus, the output of Balanced Modulator A 404 is a double-sidebandsuppressed carrier wave centered at 100 kHz. This wave is fed to SSBfilter 408, which substantially attenuates, say, the lower sideband andpasses the resulting upper-sideband SSB wave to balanced modulator B410. Balanced Modulator B is also fed by Oscillator B 412. Oscillator Boperates at higher frequency in this situation, because the USB is used.A suitable frequency would be 103.3 kHz if speech waves covering 300 to3,000 Hz are to be transmitted through lines capable of passing 300 to3,000 Hz. The output of the Balanced Modulator would then be passedthrough LPF 416 so as to attenuate undesired mixing products and the 100kHz wave. In many instances, the LPF 416 can be deleted, and othercircuits, such as amplifiers and transformers with limited frequencyresponses can be used to provide the filtering action.

The system of FIG. 4 will translate 300 Hz waves to 3,000 Hz, 1,000 Hzwaves to 2,300 Hz, and 3,000 Hz waves to 300 Hz. At the far end of thecircuit, the complementary operation is performed, restoring thefrequency of the speech components within the relative accuracy of theoscillator used in the system.

It will be apparent to those skilled in the applicable art that a phaseshift type SSB system may be substituted in the system of FIG. 4 for thefilter system shown.

In all cases, it is understood that the above described arrangements aremerely illustrative of the many possible specific embodiments whichrepresent applications of the present invention. Various changes andmodifications can be readily devised in accordance with the principlesof the present invention without departing from the spirit of theinvention and within the scope of the following claims.

What is claimed is:
 1. The method of implementing a TASI type systemutilizing transmission lines of the two wire configurationcomprising;(a) separating talk and listen signals at junctions of twowire and four wire lines and rendering crosstalk, due to imperfectseparation, unintelligible by, (b) encoding the speech transmitted inone direction differently than speech transmitted in the otherdirection, and (c) decoding the encoded speech at the far end of thesystem.
 2. The method of reducing the effect of crosstalk in a TASI typesystem, comprising;(a) encoding the transmitted speech (prior totransmission by a two wire telephone line carrying a TASI signal) atonly one end of a circuit, and, (b) decoding the encoded speech at thereceive end of the circuit after a hybrid circuit has separated talk andlisten waves so as to substantially eliminate intelligible crosstalk. 3.The method of claim 2, wherein the encoding step is performed byfrequency inversion at the transmit end of the circuit, andcomplementary frequency inversion is performed at the receive end of thecircuit so as to restore the speech wave to its normal intelligiblestate.
 4. A telephone system utilizing two wire type transmission lines,the improvement comprising;(a) means for frequency inverting speechwaves generated only at one end of the communications line, and (b)means for restoring the proper frequency characteristics of the speechwave by frequency inverting the received wave at the far end of theline.
 5. The system of claim 4 wherein the system is of the TASI type.